Thursday, December 27, 2012

A couple weeks ago a vulnerability was posted for the dlink DCS-9xx series of cameras. The author of the disclosure found that the setup application that comes with the camera is able to send a specifically crafted request to a camera on the same network and receive its password in plaintext. I figured this was a good chance to do some analysis and figure out exactly how the application carried out this functionality and possibly create a script to pull the password out of a camera.The basic functionality of the application is as follows:

Application sends out a UDP broadcast on port 5978

Camera sees the broadcast on port 5978 and inspects the payload – if it sees that the initial part of the payload contains “FF FF FF FF FF FF” it responds (UDP broadcast port 5978) with an encoded payload with its own MAC address

Application retrieves the camera’s response and creates another UDP broadcast but this time it sets the payload to contain the target camera’s MAC address, this encoded value contains the command to send over the password

Camera sees the broadcast on port 5978 and checks that it is meant for it by inspecting the MAC address that has been specified in the payload, it responds with an encoded payload that contains its password (base64 encoded)

After spending some time with the application in a debugger I found what looked like it was responsible for the decoding of the encoded values that are passed:

super exciting screen shot.

After spending some time documenting the functionality I came up with the following notes (messy wall of text):

Translated into english: the application first uses a lookup table to translate every byte in the input string, to do this it uses the value of the current byte as an offset into the table. After it is done with “stage1” it traverses the translated input buffer a dword at a time and does some bit shifting and addition to fully decode the value. The following roughly shows the “stage2” routine:

(Dword[0] << 2) + (Dword[1] >> 4) = unencoded byte 1

(Dword[1] << 4) + (Dword[2] >> 2) = unencoded byte 2

(Dword[2] << 6) + Dword[3] = unencoded byte 3

I then confirmed that this routine worked on an “encoded” value that went over the wire from the application to the camera. After confirming the encoding scheme worked, I recreated the network transaction the application does with the camera to create a stand alone script that will retrieve the password from a camera that is on the same lan as the “attacker". The script can be found here, thanks to Jason Doyle for the original finding (@jasond0yle ).